Video compression is commonly used to reduce the data storage and or transmission requirements of a recorded video stream. For example, the Motion Picture Experts Group (MPEG) standards define several commonly used video compression standards.
MPEG-1 is intended for progressive video and is commonly used to store video on compact discs, such as Video Compact Disc (VCD). The MPEG-1 standard defines a group of pictures (GOP). Referring to prior art FIG. 1, each GOP commences with an intra-coded picture frame, I. Motion compensated predictive feedback is used to compress subsequent inter-coded frames, P. Bidirectionally predicted frames, B, are coded using motion compensated prediction based on both previous and successive I or P frames. MPEG-2 adds compression support for interlaced video content.
MPEG video compression divides each individual frame into regions called macroblocks. Individual macroblocks may be predicted from neighboring frames. A discrete cosine transform (DCT) is applied to the frame to compress the frame. The resulting DCT coefficients for each macroblock are then quantized. A variable length encoder is used to encode the data.
A rate controller is used to select the quantization step size, which for a given image complexity will determine the bit rate. In addition, the quality of the image also depends upon the quantization step size. Conventionally, the bit rate, R, is modeled according to the expression: R=X/Q, where X is the total image complexity and Q is a quantization step size. Thus, the bit-rate, quantization step size, image complexity, and image quality are inter-related.
The tradeoffs in bit rate/quality that conventional MPEG encoders make is not as sophisticated as desired. For many applications, conventional MPEG encoders do not provide a fine enough level of control, particularly for single-pass MPEG encoders used in real time systems.
Therefore, what is desired is an improved apparatus, system, and method for rate control in an MPEG encoder.